Using green manures

Green manures are a simple, cheap way to improve the fertility of your garden soil; enhance its drought resistance; and suppress the germination and growth of weeds.

Green manure crops are crops grown, not to be harvested by the grower, but to be incorporated into the soil before they reach maturity to contribute to the care and feeding of the soil. It is an old technique of soil management that has unfortunately been forgotten by many farmers and gardeners who are no longer aware of the proven benefits of such crops benefits that come at the low cost of the seeds for the green manure crop.

Green manure crops contribute directly to the fertility of the garden through the supply of important plant nutrients. Legumes in particular supply a valuable amount of nitrogen since their roots form an association with soil-borne bacteria that can transform nitrogen from the atmosphere into nitrogen compounds that can be used by plants. This is quite a complicated feat and one which can save you the cost of fertilisers. Different nutrients such as phosphorus are supplied by other green manure crops.

Green manure crops contribute indirectly to nutrient supply as well. The process of decomposition of the crop aids in making further nutrients available that are already present in the soil but in a form that cannot be used by plants. It is believed that this happens through the actions of decomposition products including carbon dioxide and organic acids. An example of this indirect contribution is a barley crop. Bennett (1979) recommends growing a green manure crop of barley before a crop of tomatoes, since tomatoes have a high requirement for phosphorus and barley somehow increases the uptake of phosphorus in crops following it.

When incorporated into the soil, green manure crops can supply vast amounts of organic matter. Organic matter can also be supplied through mulches as well as through the incorporation of a green manure crop, but this usually involves greater expense. It can also be difficult to locate a source of good clean mulch such as straw that you know has not been sprayed with any chemicals, whereas, as an organic gardener, you know your crop is clean and does not contain unwanted chemical residues.

A good healthy soil should contain approximately 5% organic matter. While this may seem to be a small component of the soil, it is a vital one. According to the La Motte Soil Handbook “No other constituent plays such a major beneficial role in the soil environment and gets so little credit as does the organic fraction”. Indeed it was the emphasis placed on organic matter in the soil by the early proponents of organic growing that gave our method of agriculture its title.

Why is organic matter so important? Because decayed organic matter, or humus as it is called, is the key to soil structure, nutrient supply and the biological vitality of the soil.

The presence of humus in the soil also increases the amount of water which can be held in the soil. This is critical in making a garden drought resistant. In a dry season water applied to a garden is wasted if that water runs away and does not stay near the root zones of the plants.

Drought resistance can also be improved in another way by the use of green manure crops. Many of the legumes used as green manures, such as alfalfa, lupins and sweet clover, are very deep rooted crops. Their roots can penetrate the subsoil and open it up which is an important improvement in compacted soils. Subsequent vegetable crops can use the channels in the subsoil to allow their roots to reach deep into the subsoil and obtain water from the lower levels. It is worth remembering that many common vegetable crops are capable of putting down a large root system if the soil is loose enough. For example, in a deep, well structured soil, tomatoes can put roots down 150cm with the main root zone down to about 55cm and pumpkin and sweet corn roots can reach down to 180cm, with the main root zone down to about 60cm.

Crops can also obtain plant nutrients from the subsoil once it is opened by deep rooted green manure crops. Sourcing nutrients from these deeper levels of the soil has proved a major benefit for crops grown on farmland where the topsoil has either been eroded or has been worn out from overcropping. It is important in young gardens where the topsoil is thin. The clay subsoil in many parts of the Canberra region for instance can provide an excellent foundation for a soil building program provided it can be opened up for the crops grown in it.

Another benefit of a green manure crop is that while the green manure crop is growing it prevents weeds colonising the bare ground left after the previous crop has been removed. In general it helps protect the soil surface from erosion and leaching of nutrients.

Green manures can be grown in three ways:

As a crop during the main growing season, which, however, has the disadvantage of taking up valuable space at the most productive time of year.

As an undercover crop grown with the main crop, but planted after the main crop is established. This is an extremely useful method for gardeners in areas with long cold winters where there is not time to plant a green manure crop after the summer harvest. It is an interesting area of research in vegetable growing and for more information see Eliot Coleman’s “New Organic Grower”.

As an over winter crop, which is the most common way they are grown. In the Canberra region, autumn is an ideal time to plant green manure crops in beds emptied of the summer harvest. There is usually time to establish the crop before winter.

When establishing a garden, a green manure crop can be grown whenever a bed would otherwise be left vacant over winter. The only exception is preceding an onion crop, since onions seem to do best with no preceding green manure crop. Care should be taken with most root vegetables which do not appreciate soil with a lot of organic matter, so bulky crops should be avoided, as well as those high in nitrogen.

Once a good fertile soil has been created in the vegetable garden, it should only be necessary to replenish the supply of organic matter once in every four years, and the green manure crop can be grown at the end of a four year crop rotation such as:

Year 1: Tomatoes, capsicums, eggplants, leafy greens

Year 2: Onions, garlic or peas, beans, followed by brassicas

Year 3: Root crops

Year4: Cucurbits, Sweet corn followed by a winter green manure crop, then returning to Year 1 in the rotation.

The green manure crop can be dug in in Spring prior to planting crops for next summer. Having dug in a green manure crop you need wait only 4 to 6 weeks before planting summer crops.

However, if you consider digging in the green manure a difficult chore, you will be pleased to hear the results of a study by US Department of Agriculture scientists: Apparently they found “that amounts of nitrogen released from residues of alfalfa, wheat, and sorghum hardly differed at all whether the plants were tilled into the soil or just left there, untilled and unchopped on the surface”.

It seems therefore that you don’t have to dig it in, but incorporating it into the soil may lead to a speedier decomposition of the organic material. A satisfactory compromise is to partially chop up the crop, leave it on the soil surface, but cover it with straw to give protection to all the micro organisms who will appear to feast on the organic matter and convert it into humus in the soil.

The following Table is a list of suitable green manure crops for autumn for this region, along with brief comments which may help you decide which crop to plant. It is important to vary the types of green manure crops grown as they have different attributes and disadvantages. It is often a good idea to grow a mix of crops in the one bed to get the best results.

Autumn Green Manure Crops

Legumes: (fix nitrogen)*

Broad Beans: Produce a large amount of organic matter. Can be sown late in Autumn.